Rhodobacter sphaeroides photoproduced indole 3-acetic acid (IAA) when the precursor L-tryptophan was added to the basal medium. Of the other organic carbon sources that influenced IAA formation from L-tryptophan, alpha-ketoglutaric acid gave the maximum formation of 530 mg l(-1) in less than 24 h of illuminated anaerobic incubation. IAA was also produced from indole, glycine and glucose together by Rb. sphaeroides under photoanaerobic conditions yielding 61 mg l(-1) within 30 h.